Long persistence phosphors that emit in the green, blue-green and blue colorations have been known for some time. They include zinc-sulfide-based and strontium aluminate phosphors activated with rare earths.
However, orange and red hued long-persistence emitting phosphors have been disclosed only recently, and they have several serious drawbacks in use. For example, Royce et al, U.S. Pat. No. 5,650,094, discloses rare earth activated divalent titanate phosphors, such as CaTiO.sub.3 or Ca--Zn--Mg--TiO.sub.3, but the emission is visible only for a few minutes. Lindmayer, U.S. Pat. No. 5,043,096, reported a strontium sulfide based phosphor doped with three rare earths in the form of their oxides and fluxed with a halide, such as LiF. However, the fired fluoride phosphor was highly sintered and had to be ground to obtain a useful powdered material. However, grinding degrades the emission, and thus the phosphor must be heated or annealed to repair the crystal defect damage. However, the emission performance is never fully restored. These phosphors are described as being useful as additives to paint formulations.
Red phosphors are highly desirable because they are easy to see in the dark, particularly in the event of a power failure, for example for "EXIT" signs in a darkened theater, to mark fire fighting equipment, to make visible other safety devices and breathing masks, fire axes and the like. Safety devices and signs such as the above have long used red colors, and thus they are familiar to most persons. In addition, for decorative or novelty uses, such as for toys, automobile hub caps, sporting goods and the like, red is also a very desirable bright color.
Thus a red-emitting, long lasting phosphor has long been sought.